Molecular Sieves
Natural and synthetic zeolitic crystalline aluminosilicates are useful as catalysts and adsorbents. These aluminosilicates have distinct crystal structures which are demonstrated by X-ray diffraction. The crystal structure defines cavities and pores which are characteristic of the different species. The adsorptive catalytic properties of each crystalline aluminosilicate are determined in part by the dimensions of its pores and cavities. Thus, the utility of a particular zeolite in a particular application depends at least partly on its crystal structure.
Molecular sieves such as ZSM-5 and Y-zeolites are materials of great importance in catalytic processes. Active acid sites and shape selectivity often grant molecular sieves interesting characteristics in catalyzing chemical reactions.
Crystalline aluminosilicates are usually prepared from aqueous reaction mixtures containing alkali or alkaline earth metal oxides, silica, and alumina. "Nitrogenous zeolites" have been prepared from reaction mixtures containing an organic templating agent, usually a nitrogen-containing organic cation. By varying the synthesis conditions and the composition of the reaction mixture, different zeolites can be formed using the same templating agent.
In recent years many new breeds of molecular sieves with various pore sizes and acidity have been synthesized. Synergistic effects may be obtained by forming new materials consisting of two kinds of molecular sieves, one as a core encapsulated by another as the shell. The catalytic performance of the core will be affected by the nature of the shell in terms of shape selectivity and acidity, especially in diffusion controlled reactions.
The intergrowth phenomenon of the zeolite crystals is also known. It corresponds to a heterogeneous crystallization in which the crystals of a zeolite B appear sporadically during the crystallization of a zeolite A. Microscopic examination does not generally detect the zeolite intergrowths. These are evidenced by microdiffraction studies in which zones of the zeolite B appear as defects in the structure of the zeolite A. The best known example of intergrowth is that of the zeolite T which issues from the intergrowth of offretite and erionite. Since the intergrowth leads to a perturbation in the form and/or the size of the cages and channels, the zeolite AB obtained will have different properties from the two zeolites of which it is formed.
In general, zeolites may be divided into ten different structural types depending on the structural building blocks. These groups include the analcime group, natorlite group, chabazite group, phillipsite group, heulandite group, mordenite group, faujasite group, laumontite group, pentasil group and the clathrate group. For an overview of zeolite science and the preparation of zeolite molecular sieves, one may wish to refer to Denkewicz R. P. (1987), "Zeolite Science: An Overview," from Jrnl. Mater. Ed., 9(5) and Breck, D. W. (1984), Zeolite Molecular Sieves, R. E. Krieger Publishing Co., Malabar, Fla., both incorporated herein by reference.
Molecular sieves of the crystalline zeolite type as well as the aluminum phosphate, crystalline silicoaluminophosphate or crystalline metal aluminophosphate type are known in the art and now comprise hundreds of species of both naturally occurring and synthetic compositions. In general, the crystalline zeolites are aluminosilicate frameworks based on an infinitely extending three-dimensional network of SiO.sub.4 and [AlO.sub.4 ].sup.-1 tetrahedra linked through common oxygen atoms. The framework structure encloses cavities occupied by large ions and water molecules, both of which have considerable freedom of movement, permitting ion exchange and reversible dehydration.
The aluminum phosphate molecular sieves are generally structures comprised of [AlO.sub.4 ].sup.-1 and [PO.sub.4 ].sup.+1 tetrahedra linked through common oxygen atoms. Molecular sieves are attractive as interactive support materials because of their structural features and physical properties. These materials can provide shape selectivity, ion exchange, acid-base sites, and large electrostatic fields.
Early crystalline aluminophosphates and a method for their preparation are disclosed in U.S. Pat. No. 4,310,440, incorporated herein by reference in its entirety. The class of aluminophosphate described therein have an essential crystalline framework structure, the chemical composition of which is expressed in terms of molar ratios of oxides as: EQU Al.sub.2 O.sub.3 : 1.0.+-.0.2P.sub.2 O.sub.5,
said framework structure being microporous where the pores are uniform and in each species have nominal diameters within the range of from 3 to 10 Angstroms, an intracrystalline adsorption capacity for water at 4.6 torr and 24.degree. C. of at least 3.5 weight percent, the adsorption of water being completely reversible while retaining the same essential framework topology in both the hydrated and dehydrated state. The term "essential framework topology" refers to the spatial arrangement of the primary Al--O and P--O bond linkages. No change in the framework topology indicates that there is no disruption of these primary bond linkages. The aluminophosphates are prepared by hydrothermal crystallization of a reaction mixture prepared by combining a reactive source of phosphate, alumina and water and at least one templating agent.
For an overview of structures and template concepts of aluminophosphates see P. J. Grobet et al. (Editors) Innovations in Zeolite Materials in Science, Elsevier Science Publishers, B. V., Amsterdam. This article describes several different types of aluminophosphates.
Aluminophosphates are different from microporous compositions synthesized with silica. The aluminophosphate molecular sieves are moderately hydrophilic, apparently due to the difference in electronegativity between aluminum and phosphorus. Their intracrystalline pore volumes and pore diameters are comparable to those known for zeolites and silica molecular sieves.
One class of aluminophosphates are substituted with silicon. They are described in U.S. Pat. No. 4,440,871, incorporated herein by reference in its entirety. The materials have a three dimensional crystal framework of PO.sub.2 +, AlO.sub.2 - and SiO.sub.2 tetrahedral units, and exclusive of any alkali metal or calcium which may optionally be present, an as-synthesized empirical chemical composition on an anhydrous basis of: EQU mR:(Si.sub.x Al.sub.y P.sub.z)O.sub.2
wherein "R" represents at least one organic templating agent present in the intracrystalline pore system: "m" represents the moles of "R" present per mole of (Si.sub.x Al.sub.y P.sub.z) and "x", "y", and "z" represent the mole fractions of silicon, aluminum and phosphorus, respectively as tetrahedral oxides. Examples include,SAPO-5, SAPO-11, SAPO-16, SAPO-17, SAPO-20, SAPO-31, SAPO-34, SAPO-35, SAPO-37, SAPO-40, SAPO-42 and SAPO-49.
Other crystalline microporous compositions include crystalline metal aluminophosphates. They are described in U.S. Pat. No. 4,567,029, incorporated herein by reference in its entirety. The members of this novel class of crystalline metal aluminophosphates have a three-dimensional microporous framework structure of MO.sub.2, AlO.sub.2 and PO.sub.2 tetrahedral units and have an empirical chemical composition on an anhydrous basis expressed by the formula: EQU mR:(M.sub.x Al.sub.y P.sub.z)O.sub.2
wherein "R" represents at least one organic templating agent present in the intracrystalline pore system; "m" represents the moles of "R" present per mole of (M.sub.x Al.sub.y P.sub.z)O.sub.2 and has a value of from zero to 0.3; "M" represents at least one metal of the group magnesium, manganese, zinc and cobalt; and "x", "y" and "z" represents the mole fractions of "M", aluminum and phosphorus, respectively, present as tetrahedral oxides.
Highly crystalline cobalt aluminophosphates of type 36 have been synthesized and characterized. See "Investigations on the CoAPO-36 Molecular Sieve", Akolekar, D. B., Catalysis Letters 28 (1994) 249-262.
In an article titled "Synthesis, Characterization, Thermal Stability, Acidity and Catalytic Properties of Large-Pore MAPO-46", J. Chem. Soc. FARADAY TRAN., 1993, 89(22) 4141-4147, Akolekar, et al. the catalytic activity of MAPO-46 in ethanol to aromatics conversion is discussed.
Another class of crystalline molecular sieves have a three dimensional microporous framework structure of MgO.sub.2.sup.-2, AlO.sub.2, PO.sub.2.sup.+ and SiO.sub.2 tetrahedral oxide units. They are described in U.S. Pat. No. 4,882,038, incorporated by reference in its entirety. These molecular sieves exhibit ion-exchange, adsorption and catalytic properties. The members of the class have an empirical chemical composition on an anhydrous basis expressed by the formula: EQU mR:(Mg.sub.w Al.sub.x P.sub.y Si.sub.z)O.sub.2
wherein R represents at least one organic templating agent present in the intracrystalline pore system; "m" represents the molar amount of "R" present per mole of (Mg.sub.w Al.sub.x P.sub.y Si.sub.z)O.sub.2 and has a value from zero to about 0.3; and "w", "x", "y" and "z" represent the mole fractions of magnesium, aluminum, phosphorus and silicon, respectively, present as tetrahedral oxides.
The Mg APSO compositions are generally synthesized by hydrothermal crystallization for an effective time at effective pressures and temperatures from a reaction mixture containing reactive sources of magnesium, silicon, aluminum and phosphorus and an organic templating agent, as described in U.S. Pat. No. 4,882,038.
In an article titled "Comparison of Thermal Stability, Acidity, Catalytic Properties and Deactivation Behavior of Novel Aluminophosphate-based Molecular Sieves of Type 36", J. CHEM SOC. FARADAY TRANS. 1994, 90 (7), 1041-1046, D. B. Akolekar discusses studies in which different MeAPOS, containing Mn, Zn, Co, and Mg were prepared and characterized.
In U.S. Pat. No. 5,167,942 there is disclosed a process for preparation of faujasite-type zeolites or aluminum phosphate molecular sieves which include an encapsulated multidentate metal chelate complex which comprises preparing an aqueous alkaline admixture of aluminate and silicate anions and an alkaline or alkaline earth hydroxide, in a molar ratio and a pH appropriate for the formation of a zeolite of the faujasite group,
introducing a multidentate metal chelate complex larger than nominal pore size of the faujasite zeolite into the admixture, PA1 reacting the admixture under conditions appropriate for the formation of a crystalline zeolite of the faujasite group and PA1 preparing from the reacted admixture a crystalline zeolite having a multidentate metal complex encapsulated within the zeolite.
In U.S. Pat. No. 4,847,224 there is disclosed a binary zeolite system comprising two zeolites, A and B, having different crystalline structures while having common structural units, where the crystals of zeolite A, forming a central core, are selected from the group consisting of offretite and omega zeolites; and the crystals of zeolite B, forming the crown, are selected from omega zeolite and mordenite. Zeolites A and B being. disposed concentrically and following the same longitudinal axis are both limited to aluminosilicate compositions.
A surface-inactive shape selective metallosilicate catalyst, useful for the conversion of lower molecular weight olefins is disclosed in U.S. Pat. No. 4,868,146. The novel composition comprises an inner core portion and an outer portion disposed as a porous shell around the inner portion, wherein the inner portion consists essentially of metallosilicate zeolite having a medium pore structure, such as ZSM-5 or ZSM-23 and the outer portion comprising a fluoride containing crystalline shell consisting essentially of silica substantially free of acidic sites and having substantially the same crystalline structure as the inner core portion. This system is limited to a core and shell of the same crystal structure and XRD pattern.
U.S. Pat. No. 4,936,977 discloses a crystalline zeolite SSZ-24 which is prepared using an adamantine quaternary ion as a template and is used to convert hydrocarbons.
In U.S. Pat. No. 4,946,580 there is disclosed a method for the catalytic cracking of hydrocarbon feed to convert it essentially into gasoline and hydrogen which comprises contacting said hydrocarbon feed with a catalytically effective amount of cracking catalyst wherein the catalyst is the binary zeolite system described in U.S. Pat. No. 4,847,224, supra.
U.S. Pat. No. 5,179,054 discloses a layered catalytic cracking catalyst comprising a core and a shell, each having openings of a specified range. The shell which may further contain a metal passivator can act as a metal sink and can remove metals from the unit by attrition. The catalyst is preferably prepared by forming the core and then coating or encapsulating the core with a shell material. In this system, the core and shell configurations result from incorporating matrix and binder components with zeolite, not by directly synthesizing one zeolite around another.
In U.S. Pat. No. 5,238,676 there is disclosed a method for modifying a composition of matter comprising an inorganic, porous, non-layered crystalline phase material exhibiting, after calcination an X-ray diffraction pattern with at least one peak at a d-spacing greater than 18 .ANG. and having a benzene adsorption capacity of greater than 15 gms benzene per 100 gm of said material at 6.7 kPa (50 ton) and 25.degree. C., comprising contacting with a treatment composition comprising an inorganic oxide of a trivalent element X or a precursor of said inorganic oxide, said contacting occurring under conditions sufficient to incorporate trivalent element X in said crystalline phase material.
European Patent Application 113,473 claims the formation of a zeolite having a double structure which comprises a core made of crystalline borosilicate and a shell made of crystalline silicon oxide. The system is limited to a core and shell of the same crystal structure, i.e. showing the same X-ray diffraction pattern. There is no evidence presented on the actual forming of the core and shell configuration.
None of these references discloses a binary molecular sieve having a zeolite core and an aluminum phosphate molecular sieve shell.
FCC
Fluidized Catalytic Cracking (FCC) is well known in the refining industry as particularly advantageous for conversion of heavy petroleum fractions to lighter product.
The heavy feed contacts hot regenerated catalyst and is cracked to lighter products. Carbonaceous deposits form on the catalyst, thereby deactivating it. The deactivated (spent) catalyst is separated from cracked products, stripped of strippable hydrocarbons and conducted to a regenerator, where coke is burned off the catalyst, with air, thereby regenerating the catalyst. The regenerated catalyst is then recycled to the reactor. The reactor-regenerator assembly are usually maintained in heat balance. Heat generated by burning the coke in the regenerator provides sufficient thermal energy for catalytic cracking in the reactor. Control of reactor conversion is usually achieved by controlling the flow of hot regenerated catalyst to the reactor to maintain the desired reactor temperature.
The design of many modern FCC units provides for the addition of the hot regenerated catalyst at the base of a riser reactor. Fluidization of the solid catalyst particles is promoted with a lift gas. Steam is used to promote the mixing and atomization of the feedstock.
Hot catalyst (650.degree. C.+) from the regenerator is mixed with preheated (150.degree.-375.degree. C.) charge stock. The catalyst vaporizes and superheats the feed to the desired cracking temperature usually 450.degree.-600.degree. C.
Coke deposits on the catalyst and the feed is cracked during the upward passage of the catalyst and feed.
The coked catalyst and the cracked products exit the riser and enter a solid-gas separation system, e.g., a series of cyclones, at the top of the reactor vessel. The cracked products pass to product separation. Typically, the cracked hydrocarbon products are fractionated into a series of products, including gas, gasoline, light gas oil, and heavy cycle gas oil. Some heavy cycle gas oil may be recycled to the reactor. The bottoms product, a "slurry oil" is conventionally allowed to settle. The catalyst rich solids portion of the settled product may be recycled to the reactor. The clarified slurry oil is a heavy product.
Good overviews of FCC process can be found in: U.S. Pat. Nos. 3,152,065 (Sharp et al.); 3,261,776 (Banman et al.); 3,654,140 (Griffel et al.); 3,812,029 (Snyder); 4,093,537, 4,118,337, 4,118,338, 4,218,306 (Gross et al.); 4,444,722 (Owen); 4,459,203 (Beech et al.); 4,639,308 (Lee); 4,675,099, 4,681,743 (Skraba) as well as in Venuto et al., Fluid Catalytic Cracking With Zeolite Catalysts, Marcel Dekker, Inc. (1979) incorporated by reference herein in their entirety.
FCC catalyst can contain finely divided acidic zeolites comprising, e.g., faujasites such as Rare Earth Y (REY), Dealuminized Y (DAY), Ultrastable Y (USY), Rare Earth Containing Ultrastable Y (RE-USY), Si-Enriched Dealuminized Zeolite Y (LZ-210) disclosed in U.S. Pat. Nos. 4,711,864, 4,711,770 and 4,503,023, all of which are incorporated herein by reference) and Ultrahydrophobic Y (UHP-Y).
FCC catalysts are typically fine particles having particle diameters ranging from 20 to 150 microns and having an average diameter around 60-80 microns.
Though many improvements have been made in the FCC process, a number of problem areas remain. In addition, some process variables change depending upon the desired products. None of the available references suggests using a binary sieve containing a zeolite core and an ALPO shell in catalytic cracking.
Propylene Upgrading
As is well-known to those skilled in the art, the advent of reformulated gasolines to meet ever increasing environmental and other requirements is reflected in a significant increase in the demand for isobutylene and isoamylenes which are used to prepare methyl t-butyl ether (MTBE) and methyl t-amyl ether (TAME)--the gasoline additives of significant current interest. Isobutane and n-butenes are also of increasing importance due to the high octane alkylates that can be produced from them.
On the other hand, there are abundant supplies of propylene which are available from refining processes such as catalytic cracking. It would be desirable to be able to convert these propylene (C.sub.3.sup.=) streams to isobutane (i-C.sub.4), n-butenes (n-C.sub.4.sup.=), isobutylene (i-C.sub.4.sup.=) and isoamylenes (i-C.sub.5.sup.=) streams.
U.S. Pat. No. 4,465,884 teaches a process of converting C.sub.3+ olefins to product comprising non-aromatic hydrocarbons of higher molecular weight than feedstock olefins and aromatic hydrocarbons using large pore Y and beta zeolites. Butenes, isoamylenes and isobutane were not the products of interest.
U.S. Pat. Nos. 4,957,709 and 4,886,925 teach a system combining olefin interconversion (upgrading olefins into streams rich in isobutylene and isoamylene with the production of MTBE and TAME).
U.S. Pat. No. 5,146,029 teaches olefin interconversion by MCM-22 zeolite. The application is limited solely to the MCM-22.
U.S. Pat. Nos. 5,134,241 and 5,134,242 teach olefin upgrading using the MCM-41 zeolite.
U.S. Pat. No. 4,899,014 discloses olefins upgrading using ZSM-5, however the upgrading is mainly for gasoline production.
U.S. Pat. No. 4,556,753 teaches upgrading propylene to isobutene using silicalite zeolites in the presence of steam, however isoamylenes were not included.
U.S. Pat. No. 4,527,001 discloses small olefin interconversions using metal phosphate molecular sieves, such as, for example, AlPO.sub.4, SAPO, FeAPO and CoAPO.
Instant Invention
It would be a distinct advance in the art if it were possible to synthesize a molecular sieve with different layers of compositions, acidity and structures and take advantage of the reactant, product, and transition-state shape selectivities.
No references have been found in the art which disclose a catalyst composition comprising a binary sieve wherein the core is a crystalline zeolite and the shell is an aluminum phosphate molecular sieve (ALPO), a silicon-substituted aluminophosphate molecular sieve (SAPO), or a crystalline metal aluminophosphate (MeAPO). Furthermore, there is nothing in the art which teaches or suggests using such catalyst composition for catalytic cracking and propylene upgrading.